Hithertofore, it has been generally practiced to leach copper from copper electrolysis slime with sulfuric acid when gold, silver, etc. are recovered from said slime. In such leaching, however, tellurium, arsenic, etc. are simultaneously leached. Tellurium adversely affects electrolytic refining and electrowinning of copper and, therefore, the leach (containing tellurium) cannot be returned to the copper electrolysis stage. Thus, tellurium is removed from the leach by decoppering electrolysis in which tellurium, arsenic, etc. are precipitated together with copper or by precipitating tellurium together with arsenic, etc. with the addition of an alkali.
Another practice has been to remove and recover tellurium in the leach as copper telluride by cementation, that is, by addition of copper powder or copper pieces to the leach. When the tellurium content of the slime is high, however, it is difficult to deposit the tellurium with copper pieces because the surface area of the pieces is small. Therefore, copper powder must be used.
In the electrowinning of copper, when copper ores are leached, slight amounts of selenium and tellurium are simultaneously leached. The selenium and tellurium can be removed by passing the leach through a column packed with copper powder or pieces. However, copper powder is not practically used because:
(1) Copper powder is expensive and a considerably excess amount of copper powder is required in order to sufficiently deposit tellurium; PA1 (2) The process is inevitably of batch type and, therefore, large scale equipment is required and the equipment investment is large.
Even if copper powder is used, it is difficult to efficiently recover high grade (not less than 50% tellurium) copper telluride precipitate and to reduce the tellurium content in the solution after deposition to several ppm or less before disposal.
We have found that one cause of slowing of the cementation speed is that the surface of the copper powder or pieces is covered by deposited copper telluride in the course of cementation and found that the problems of the prior art can be overcome by using copper plate instead of copper powder.